cpusched: Fix compiler warnings

Fix the following compiler warnings that occur when building with "-W
-Wall -Wextra":

cpusched.c: In function ‘get_appropriate_cpu’:
cpusched.c:28:16: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
cpusched.c:29:23: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>